Long optics life is extremely important and extremely difficult to achieve in vacuum ultraviolet light (“VUV”) and other deep ultraviolet (“DUV”) and extreme ultraviolet (“EUV”) applications. Requirements, e.g., of >12 billion pulse optic lifetimes can create an extreme need for contamination control measures, e.g., in the design of beam delivery units (“BDUs”), i.e., methods and apparatus for delivering the output laser light pulse beam to a manufacturing tool, e.g., a micro-lithography exposure illumination tool, and monitoring, preserving and modifying beam qualities prior to such delivery.
Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric discharge gas laser and they have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled COMPACT EXCIMER LASER. This patent has been assigned to Applicants' employer, and the patent is hereby incorporated herein by reference. The excimer laser described in U.S. Pat. No. 5,023,884 is a high repetition rate pulse laser.
These excimer lasers, when used for integrated circuit lithography, are typically operated in an integrated circuit fabrication line “around-the-clock” producing many thousands of valuable integrated circuits per hour; therefore, down-time can be very expensive. For this reason most of the components are organized into modules which can be replaced within a few minutes. Excimer lasers used for lithography typically must have its output beam reduced in bandwidth to a fraction of a picometer. This “line-narrowing” is typically accomplished in a line narrowing module (called a “line narrowing package” or “LNP” for KrF and ArF lasers) which forms the back of the laser's resonant cavity (A line selection unit “LSU” is used for selecting a narrow spectral band in the F2 laser). The LNP is comprised of delicate optical elements including prisms, mirrors and a grating. Electric discharge gas lasers of the type described in U.S. Pat. No. 5,023,884 utilize an electric pulse power system to produce the electrical discharges, between the two elongated electrodes. In such prior art systems, a direct current power supply charges a capacitor bank called a “charging capacitor” or “C0” to a predetermined and controlled voltage called the “charging voltage” for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts in these prior art units. After C0 has been charged to the predetermined voltage, a solid state switch is closed allowing the electrical energy stored on C0 to ring very quickly through a series of magnetic compression circuits and a voltage transformer to produce high voltage electrical potential in the range of about 16,000 volts (or greater) across the electrodes which produce the discharges which lasts about 20 to 50 ns.
It is important not only to carefully monitor and control parameters of the laser output light pulse beam, but to deliver that light, e.g., to a manufacturing tool, often located across a room or even on a different floor in a manufacturing facility with proper light parameter characteristics which might be altered by the apparatus and method of such delivery, and therefore need monitoring and perhaps even modification as part of the deliver system and method.
A BDU system, e.g., can be a purged gas tight system. Periodically module replacement may be required for various reasons. Exposure to open air is detrimental to optics life, i.e., when the purge environment is breached. It is highly desirable to be able to remove and replace an optics module in the BDU system without contaminating (exposing) other modules to open air while the module is being removed and/or replaced. It is further desirable to be able to do such repair/replacement without then having to repurge the entire BDU.
During, e.g., the fabrication of a BDU, it may become necessary, e.g., to optically align the BDU, including, e.g., its internal modules and/or the input and output ports of the entire BDU with an external alignment methodology. There are occasions where the previous methods of aligning the BDU and/or the BDU modules using a prior, e.g., total station technique may not be feasible, e.g., due to space limitations, i.e., close proximity to other elements of the laser system or surrounding structures or machinery and/or small working areas, e.g., prohibiting open beam alignment.
The BDU system, e.g., when operating at 193 um wavelength can have a hard time achieving optics element/module long lifetimes, e.g., due to the effects of contamination on the optics. One of the biggest problems confronted by the BDU system design can be, e.g., in-situ beam alignment. In-situ, open beam alignment, e.g., can be very detrimental to optics life.
Some lasers, e.g., XLA lasers may have uneven spatial coherence, which is a problem that needs to be corrected.
Long optic life can be extremely important in DUV, VUV and EUV light source and delivery systems, including e.g., excimer or molecular fluorine gas discharge lasers with associated beam delivery units. Requirements for lives exceeding 12 B pulses can create extreme need for contamination prevention, detection and control measures in the design, e.g., of the BDU in order to achieve the lifetime requirements.